Analysis by analytical ultracentrifugation indicates that actin oligomers (approximately tetramers) form from monomeric ATP-actin below its critical concentration. Slightly above its critical concentration in the absence of free MG-ATP, about 50% of the actin may be converted to small oligomers, approximately dimers, which still contain actin-bound ATP. At steady state in the presence of F-actin, most of the unpolymerized actin appears to be small oligomers, approximately dimers to tetramers, as determined in the analytical ultracentrifuge after clearance of F-actin. These results are consistent with Oosawa's theory for the polymerization of helical polymers. Actobindin, an 88-amino acid protein from Acanthamoeba that is a potent inhibitor of actin polymerization, has been shown to form a ternary complex with 2 actin monomers. The first actin binds with a K(D) of about 2-5 (mu)M and the second actin binds either with the same K(D) but with negative cooperativity or with a significantly lower K(D).